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Review Drop Index Recommendations

The Performance Advisor suggests dropping unused, redundant, and hidden indexes to improve write performance and increase storage space.

See also

To learn more about the impact of unnecessary indexes, see Remove Unnecessary Indexes.

Unused Indexes

An index is unused if it doesn’t support any queries.

The Performance Advisor flags an index as unused if it hasn’t supported a query in 7 or more days after it was created or the server was restarted.

Redundant Indexes

An index is redundant if another index supports any query that it could.

The Performance Advisor flags an index as redundant if it matches the prefix of another index in the same collection.

Example

If a collection contains the indexes:

• { a: 1 }
• { b: -1 }
• { a: 1, b: -1 }

{ a: 1 } is redundant because it matches the prefix { a: 1, b: -1 }.

{ b: -1 } is not redundant because it does not match any prefix.

In the Performance Advisor, redundant indexes are marked with a red Redundant badge. Below each redundant index, the Performance Advisor displays the related indexes that cover it.

Note

Related indexes are displayed for you to verify that the redundant index can be dropped safely. Related indexes are not recommended for removal.

Hidden Indexes

New in MongoDB version 4.4

Hidden indexes aren’t used to support queries. They still impact write performance and consume storage space.

Hiding an index is useful for evaluating the impact of removing an index before doing so. Unhiding an index is much easier than rebuilding a deleted index.

See also

To learn more about hidden indexes, see Hidden Indexes in the MongoDB manual.

To hide an index with the Performance Advisor, see Drop or Hide an Index.

To unhide an index, see Unhide an Existing Index in the MongoDB manual.

The Performance Advisor always recommends dropping hidden indexes. After you are confident a hidden index is unnecessary, drop it.

Drop or Hide an Index

Note

MongoDB recommends hiding indexes before you drop them. Hidden Indexes are available in MongoDB version 4.4 and higher.

To drop or hide an index with the Performance Advisor:

1

Navigate to the Performance Advisor Drop Indexes recommendations.

In the Performance Advisor tab, click View Recommendations on the Drop Indexes card.

2

On the index you want to drop or hide, click Drop Index.

The Performance Advisor displays a dialog with a link to Data Explorer and a copyable MongoDB Shell command to drop that index.

In MongoDB 4.4 and higher, the dialog also provides a copyable MongoDB Shell command to hide that index.

3

Drop or hide the index through Data Explorer or MongoDB Shell.

To drop an index with Data Explorer, click the Indexes tab, then click Drop next to the index you want to drop. You will be asked for confirmation.

To drop or hide an index with MongoDB Shell, paste and run the command provided by the Performance Advisor.

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